79-84 mercury 9.8 thunderbolt

[Haydanski]

Hello, I'm new to the forum.
I recently purchased a job boat with said motor attached. Motors not running great. It's a really pain to start when cold and once you get to about half throttle it wants to die, but once it revs through that it rips. Runs fine up top and idling. I took the carb apart and blasted cleaner through every hole I could find. The fuel pump gaskets are definitely old but I've seen worse. I have a kit on order as well as a new impeller. I've only worked on small lawn equipment. Hoping someone with some more experience could give me some pointers.
Thank you

 

[keithb7]

I’ll follow with interest as I own a few same era 9.8 Mercs. I find the carbs to be very picky and sensitive. They want to be dial’d-in just right.

One of my 9.8’s when cold will not start. No amount of choke or repeated pulling helps. It refuses to fire unless I inject a shot of raw fuel into the carb intake. Then it fires right up and runs beautifully through all rpm ranges. I can shut it off and restart it as often as I like. It runs great….Let it cool down for 45 mins? No go. It wants a shot of raw fuel again. So I carry a small syringe with me. She gets her shot and everyone is happy. I suspect its from the measured 75 psi compression when cold. It can’t suck enough fuel in. When the engine is running or warmed up and rings/pistons expand, the compression is up. Then she can suck hard enough to draw fuel.

Yours symptoms lead my mind to mid range fuel delivery issues. Perhaps a hole or 2 is plugged up in the brass diffuser? Did you take it out and have a look at it? Does the inner black plastic venturi look like its all there, intact? Have you checked your float level?